Tracing device utilizing line-scanning photoelectric pick-up means



p 1965 F. J. RAU ETAL 3, 0 67 TRAGING DEVICE UTILIZING LINE-SCANNINGPHOTOELECTRIG PICK-UP MEANS Original Filed Sept. 15, 1958 4 Sheets-Sheet1 Sept. 28, 1965 F. J. RAU ETAL TRACING DEVICE UTILIZING LINE-SCANNINGPHOTOELECTRIC PICK-UP MEANS Original Filed Sept. 15, 1958 4 Sheets-Sheet2 TDI Traclng Drive Moior H n w U y s u M 2 I R R u R R w R O R W F F M0 O 0 o C o 3 w a S h D.. e r P U A Discringipuior INVENTOR Frank J. Ruu8 Kenneth Losch a Y I! ATTORNEY Servo Amplifier Fig. 2

Pulsuhon Amplifier Se t. 28, 1965 F. J. RAU ETAL TRACING DEVICEUTILIZING LINE-SCANNING PHOTOELECTRIC PICK-UP MEANS Original Filed Sept.15, 1958 4 Sheets-Sheet 3 mmk NmP fin Q FEE-S 01 Sept. 28, 1965 F. J.RAU ETAL 3,209,267

TRACING DEVICE UTILIZING LINE-SCANNING PHOTOELECTRIC PICK-UP MEANSOriginal Filed Sept. 15, 1958 4 Sheets-Sheet 4 Discriminuior AmplifierServo Amplifier United States Patent 3 Claims. (Cl. 328146) Thisinvention is a division of an application Serial No. 759,243 filedSeptember 15, 1958, now abandoned, and assigned to Westinghouse ElectricCorporation.

This invention relates to the tracer art and to the amplifier art andhas particular relationship to line tracers such as disclosed inCheverton Patent 2,633,612 in which an industrial tool such as a cuttingtorch or a profiler is caused to follow a curve and to amplifiers havingparticular applicability to preventing line tracers from producingexcessive scrap by maloperation of such tracers.

Cheverton patent relates to a tracer in which an industrial tool iscaused to follow a curve formed by a line drawn in pencil or ink onpaper or like material. The Cheverton apparatus includes a photoelectricpick-up device which produces a pulsating signal having a wave formdependent on the instantaneous position of the pickup relative to theline being followed. This signal controls a motor which tends to resetthe pick-up so that it moves precisely along the line. The pick-up isconnected through a pantagraph to the tool and the tool follows thepick-up and is controlled by the curve.

The Cheverton apparatus has been constructed and operated and hasproduced highly satisfactory results. But this apparatus is not entirelyautomatic. The curve which is traced is usually closed. The Chevertontracer would then cause the tool to move continuously in accordance withthe closed curve unless stopped by an attendant. This manner ofoperation of the Cheverton apparatus demands that the attendant devoteattention continuously to the operation.

It is an object of this invention to provide tracer apparatus forcontrolling the movement of a tool in the operation of which the toolshall automatically come to a stop once the curve controlling the toolhas been completely traced and specifically to provide an amplifier forachieving such selective stopping.

In accordance with this invention an amplifier is provided for tracerapparatus in the operation of which the movement of the tool continuesonly so long as the photoelectric pick-up is scanning a line and thetool is stopped while the pick-up is not scanning a line. This apparatusfurther includes manually actuable means for operating the apparatusindependently of the line so long as the manually actuable means isactuated. Thus the operation of the apparatus may be started in theabsence of a line by the manually actuable means and may be stopped inthe absence of a line if the manually actuable means is unactuated. Theamplifier according to this invention effectuates this operation.

In the usual practice of this invention the curve being scanned isprovided with a gap. The operation is started near one terminal of thegap by the manually actuable means. Once the photoelectric pick-up picksup the line, the operation continues with the pick-up moved along thecurve away from the gap independently of the manually actuable meansuntil the pick-up returns to the gap. The manually actuable means may beactuated long enough to enable the pick-up to pick up the line and thenreleased. When the pick-up returns to the gap, the

3,26%,257 Patented Sept. 28, 1965 operation stops because the manualmeans is then unactuated.

In using apparatus in accordance with this invention a single operatormay supervise the operation of a number of tool-tracer combinations. Theoperator starts each combination in its turn actuating the manual meansand once it starts moves on to the next combination. Each combinationonce started continues to operate until the pick-up returns to the gap.At this point the operation stops until the operator returns and startsa new operation.

In addition to the facility for completely automatic operation theapparatus embodying this invention has the advantage that in anysituation in which the pick-up inadvertently moves to a position inwhich there is no line the apparatus stops, thus excessive scrap isavoided.

In accordance with the specific aspects of this invention the means forstopping the apparatus in the absence of a line being traced iscontrolled by the pulsating signal received from the curve being traced.So long as this pulsating signal continues the pick-up and the toolfollowing it continues to operate. When the pulsations are interruptedthe movement of the pick-up and tool is stopped. So long as there isreasonable light contrast between the line and the surface on which itis drawn, the stopping of the pick-up tool operation is independent ofthe amount of light that may be picked up by the pickup. Thus the stopmechanism is within reasonable limits etfective regardless of thecharacteristic of the surface on which the curve being scanned by thepick-up is produced.

The novel features considered characteristic of this invention aredisclosed generally above. The invention itself both to its organizationand as to its method of operation together with additional objects andadvantages thereof will be understood from the following description ofan embodiment when taken in connection with the accompanying drawings inwhich:

FIGURE 1 is a block diagram of a preferred embodiment of this invention;

FIG. 2 is a circuit diagram in FIG. 1;

FIG. 3 is a diagrammatic presentation of the manner in which theinvention is applied to the cutting out of a plurality of slabs ofdiiferent shapes from a plate; and

FIG. 4 is a circuit diagram similar to FIG. 2 but showing the magnitudesof the important components used in a circuit in accordance with thisinvention which has been constructed and found to operatesatisfactorily.

FIG. 4 is presented only for the purpose of aiding those skilled in theart in practicing this invention and not with the intention of limitingthe invention in any Way.

The apparatus in accordance with this invention includes a PICK-UP UNIT,a TOOL UNIT, a PANTO- GRAPH and a CONTROL UNIT. The PICK-UP UNIT may besimilar to that disclosed in the Cheverton patent. So that therelationship between the apparatus disclosed herein and in the Chevertonapparatus may be understood, the labeling of the Cheverton appiicationhas been adapted to the extent practicable in the figures of thisapplication.

The PICK-UP UNIT includes a plate 7 on which is mounted a scanning head8. The scanning head 8 includes a photo cell 24 which is vibrated withreference to the line 20 of the curve being scanned. The pick-up alsoincludes a drive-wheel mechanism 9 and a drive 10 includ ing a steeringmotor 38 and a tachometer generator 39 which produces a signalproportional to the speed of the motor 38. The drive-wheel mechanismincludes a rotatable tube 36 which carries the wheel 42. The motor 38,through a gear train 11, 12, 13, 14, 15, 16, 17, rotates the tube 36 andthe scanning head 8. The wheel 42 is mounted on a shaft (not shown)which is driven by the tracing of the embodiment shown 3 drive motor 37.The steering motor 38 is controlled from a servo amplifier 44 in theCONTROL UNIT which is in turn controlled from the photo cell 24. Anegative feedback signal is supplied from the tachometer to the servoamplifier to suppress any tendency to overshoot by reason of excessivevelocity or acceleration of the steering motor.

While this invention may be used to control tools of different types, itis shown in its specific aspects applied to control cutting torches. TheTOOL UNIT includes a plurality of gas cutting torches TR1, TR2, TR3.Each of these torches is of the acetylene type and is provided with atube 101 through which the cutting gas is projected and with theconventional gas mixer 103. Each of the mixers is supplied from amanifold controlled by suitable valves V1, V2, V3. The manifold includesa cutting-oxygen conductor 105 controlled by valve V1, a paddlingoxygenconductor 107 by valve V2 and an acetylene conductor 109 by valve V3.The puddling-oxygen is supplied at a relatively low rate and .thecutting oxygen at a relatively high rate. The valves V1, V2, V3 aremagnetically controlled and may be opened by energizing solenoids L1,L2, L3, respectively (FIG. 2). The torches TRll, TR2, TR3 of the TOOLUNIT are connected to the PICK-UP UNIT through the PANTOGRAPH which maybe of the conventional mechanical type. Thus, the movement of thetorches is controlled through the PICK-UP UNIT and this in turn iscontrolled by the line so that the torches move corresponding to thecurve of the line 20.

The CONTROL UNIT is energized from a direct-current supply and includesin addition to the servo amplifier a pulsation amplifier, a rectifier, adiscriminator amplifier, and a relay unit. The relay unit is connectedto control the valves V1, V2 and V3 through the solenoids L1, L2, L3 andthe tracing drive motor 37.

The pulsation amplifier (FIG. 2) which constitutes the subject of thisinvention includes a discharge device 1TU which may be part of a doubletriode and which has an anode 111, a cathode 113, and a grid 115. Thecathode 113 is grounded through a relatively low resistor 117. The grid115 is grounded through a grid resistor 119 and is also connected toground through the photoelectric device 24. The anode is connected tothe positive terminal of .the supply through an anode resistor 121.Through another resistor 123 a potential is impressed across the photocells 24.

The output of the pulsation amplifier is derived from the anode circuitof ITU through a coupling circuit including a capacitor 125 and theprimary IPO of a transformer 1TC which are connected between the anode111 and ground. This coupling network transmits only the alternatingcomponents of any signal appearing on the anode of ITU and thustransmits only the pulsations produced by the oscillations of the photocell 24 with reference to the line 20. The rectifier labeled RX is ofthe dry-cell bridge type and is connected to the secondary 15C of thetransformer lTC. The transformer 1TC has a high transformation ratio sothat the potential across llSC is approximately ten times the potentialacross 1P0.

The discriminator amplifier includes a discharge device 2TU which may bethe section of the double triode which includes 1TU and which has ananode 131, a cathode 133, and a grid 135. The discriminator amplifieralso includes a time-constant network N consisting of at capacitor 137and a resistor 139. This network N is connected through a resistor 141between the output terminals of the rectifier RX. The discriminatoramplifier also includes a biasing network B supplied with a potentialfrom an alternating-current supply through a rectifier RX1 and includinga capacitor 143 shunted by a variable resistor 145. The cathode of 2TUis grounded through a relatively low resistor. The anode 131 of 2TU isconnected to the positive terminal of the direct-current supply throughthe coil of a relay R1 of the relay unit. The cathode 133 is groundedthrough a low resistor 151. The grid 135 is connected to ground throughthe network N and the 4 biasing network B. The network B is so poled asto tend to block conduction of 2TU and the network N is poled to permitconduction of 2TU. Thus, for any setting of B, N is capable of causingconduction of 2TU when the potential across it exceeds a predeterminedmagnitude.

The potential of network N is dependent on the contrast between the line20 being traced and the area n which the line is impressed. By properlysetting B the conduction of 2TU may be so controlled that the relay R1is actuated when the scanner 8 is scanning a line drawn to control theTOOL UNIT but does not conduct for the spurious variations arising fromsurface imperfections of the surface on which the line 20 is impressed.

The Relay Unit includes, in addition to the relay R1, a relay R2 andtime-delay relays TD1 and TD2. The Relay Unit also includes a pushbuttonPB and indicating lamp IL and manual switches S2, S3, S4, S5 and S6 forsetting .the apparatus during operation. S6 and S5 permit operation ofmotor 37 or supply f cutting acetylene independently of each other.

Relay R1 has a front contact RM and a back contact R112. Relay R2 hasfront contacts R2a and contacts R2b. Relay TD1 has a front contact TDla;relay TD2 a front contact TD2a. Relay TD1 and TD2 are of the type whichpull in instantaneous when current is applied to their coils and dropout only a predetermined time interval after the current through thecoils is interrupted.

The Relay Unit is supplied from an alternating current commercial supplyof the usual volts 60 cycle type through conductors AL1 and AL2. Thecoil of R2 is adapted to be connected between conductors AL1 and AL2through the pushbutton PB. This coil is also adapted to be connectedbetween conductors AL1 and AL2 through front contact R1a. Back contactRlb connects the lamp IL between conductors AL1 and AL2. This lampindicates that the scanner head 8 is not scanning a line.

The tracer drive motor 37 (FIG. 1) is adapted to be connected betweenconductors AL1 and AL2 through the front contact R2a of relay R2.Solenoid L1 which controls valve V1 is adapted to be connected betweenconductors AL1 and AL2 through front contact R2a and a switch S2 whichmust be closed to energize the solenoid L1. Solenoid L2 is adapted to beconnected between conductors AL1 and AL2 through front contact TDla:Solenoid L3 is adapted to be connected between conductors AL1 and AL2through front contact TD2a. The coils of relay TD1 and TD2 are adaptedto be connected between conductors AL1 and AL2 through front contact R2band through switches S4 and S5, respectively. These coils are alsoadapted to be connected in the same way through switch S3 which is openfor automatic operation and closed for manual operation.

FIG. 3 shows a setup for using the apparatus in accordance with thisinvention for a cutting operation. In this view the PICK-UP UNIT and theTOOL UNIT are connected through the PANTOGRAPH. The PICK-UP UNIT isdisposed with the scanner 8 over the area where the curves 201, 203, 205to be followed are impressed. The TOOL UNIT is disposed with the torchesTRl, TR2, TR3 over the plate 211 from which slabs 213, 215, 217corresponding to the curves 201, 203, 205, respectively, are to be cut.In the practice of the invention, the scanner head 8 and the othercomponents of the PICK-UP UNIT follow each curve 201, 203, 205 in itsturn and the torches TR1, TR2, TR3 following the head 8 cut out theslabs 213, 215, 217 of the desired form. For each curve being traced theoperation is automatic once it is started by closing the pushbutton PB.

In the standby condition of the apparatus the conductors AL1 and AL2 areenergized and the direct-current supply for the pulsation amplifier,rectifier, and discriminator amplifier are energized. The pushbutton PBand the switches S2, S3, S4 and S5 are open. The photo cell isvibrating, and by reason of the vibrations produces small variations inresponse, but the input circuit of discharge device 1TU is not beingsupplied with signal potential large enough to charge network Nsuflicient to overcome the bias B and device 2TU is maintainednonconducting. Relay R1 is then deenergized and the indicating lamp ILis energized.

Let it be assumed that the slabs 213 on the left of the plate 211 inFIG. 3 are to be cut out. The PICK-UP UNIT is then set so that thescanning head 8 will scan a point just beyond the tail 221 of the curve201 on the left. In this position of the PICK-UP UNIT the torches TR1,TR2 and TR3 are set in a position from which they can conveniently cutout the slabs 213 corresponding to the curve 201. The PICK-UP UNIT isnow energized so that the photo cell 24 is vibrated and switches S4 andS5 are closed. At this point, the cell 24 is not scanning a line anddischarge device 2TU continues deenergized so that relay R1 isdeenergized. The tracing drive motor 37 is then not energized and theapparatus is still in the quiescent condition.

To start an operation the scanner 8 is positioned so that on moving itwould pick up the tail 221 of curve 201 and switches S2, S3, S4, S5 andS6 are closed. The closing of S3 energizes relays TD1 and TD2 so thatvalves V2 and V3 are opened supplying acetylene and puddling oxygen.This gas mixture is ignited and the work heated to a near molten state.The puddling operation is now complete.

The pushbutton PE is now closed energizing R2 and the tracing drivemotor 37 and opening valve V1 so that cutting oxygen is supplied. Thescanner 8 now picks up the line and at the same time the torches TRll,TR2, TR3 start a cutting operation.

Discharge device 2TU and relay R1 are then energized. The scanner 8 andwith it the torches will follow the tail 221 and on reaching the gap 225will cross the gap in a straight line rather than following the cusp ofthe curve 201 at the gap. When scanner 8 again picks up the line relayR2 is locked in at contact Rla and the operator may release thepushbutton PB. The operator now opens switch S3 and turns to the nextcutting assembly. The scanning head and the TOOL UNIT continue to followthe curve on the left and three slabs corresponding to the curve are cutout.

While the operator is engaged in starting the next operation, theapparatus shown in FIG. 3 continues to operate with the scanning head 8scanning the left hand curve 201 and the torches TRl, TR2 and TR3cutting out three slabs 213. This operation continues until the scanninghead 8 again scans the gap 225 in the curve. At this point, thepulsating potential in the input of ITU is interrupted and the dischargeof network N starts. The gap 225 is of such length that network Ndischarges before the scanning head 8 has passed over the gap. Dischargedevice 2TU is then deenergized, deenergizing relay R1 and energizing theindicating lamp IL. Relay R2 is then deenergized, deenergizing thetracing drive 37. In addition, solenoid L1 is immediately deenergizedand solenoids L2 and L3 are deenergized after TD1 and TD2 time out.Valve V1 is first closed and is followed by valves V2 and V3 and themovement of the PICK-UP UNIT and the current is stopped. An operation isnow completed.

When the operator is ready he may now start another operation in themanner just disclosed. The first operation was carried out while theoperators attention was devoted to other apparatus.

In the case of curve 203 there is no gap in the curve. The movement ofthe scanner 8 starts at the tail 231 and follows the rectangles a, b andc in succession. The scanner passes from rectangle c to line 233 andstops at the end of this line. In this case the pushbutton PB may bereleased and S3 opened as soon as the scanner 8 picks up the tail 231.Rectangles 203 may be used in chain cutting. Three rectangles are cutout automatically.

While a preferred embodiment of this invention has been disclosedherein, it is realized that many modifications thereof are feasible.This invention, then is not to be restricted except insofar as isnecessitated by the spirit of the prior art.

We claim as our invention:

1. An amplifier for use with apparatus which operates responsive torepeated potential variations having generally predetermined periods asdistinct from spurious potential variation, the operation of saidapparatus to be stopped on the disappearance of said repeated potentialvariations, the said amplifier comprising an input valve having acontrol electrode and principal electrodes, means connected to saidprincipal electrodes for impressing a substantially non-varyingenergizing potential between said principal electrodes, means connectedto said control electrode and one of said principal electrodes forimpressing both said repeated variations and said spurious variationsbetween said control electrode and said one principal electrode, saidrepeated variations and said spurious variations being impressedtogether when said repeated variations are present and said spuriousvariations being impressed alone when said repeated variations are notpresent, a time-constant network having a time constant substantiallylonger than the longest of said periods of said repeated variation,means connecting said principal electrodes to said network, saidconnecting means including means for suppressing any direct currentcomponent of the potential appearing across said principal electrodesand also including means impeding the flow of current from saidprincipal electrodes to said network so that potential of a substantialmagnitude dependent on the amplitude of said repeated variations isbuilt up in said network when said repeated variations are appreciablypresent and the potential on said network remains low in the absence ofsaid repeated variations, an output valve having a control electrode andprincipal electrodes, means connected to said principal electrodes ofsaid output valve for impressing between said principal electrodes anonvarying energizing potential, and means connecting said networkbetween said control electrode and one of said principal electrodes ofsaid output valve to impress the potential on said network incontrolling relationship between said last-named control electrodes andprincipal electrode, said network being so connected that the potentialof said substantial magnitude built up on said net- WOIk, when saidrepeated variations are appreciably present, maintains said valveconducting, and said valve being non-conducting when repeated variationsare absent.

2. An amplifier for use with apparatus which operates responsive torepeated potential variations having generally predetermined periods asdistinct from spurious potential variation, the operation of saidapparatus to be stopped on the disappearance of said repeated potentialvariations, the said amplifier comprising an input valve having acontrol electrode and principal electrodes, means connected to saidprincipal electrodes for impressing a substantially non-varyingenergizing potential between said principal electrodes, means connectedto said control electrode and one of said principal electrodes forimpressing both said repeated variations and said spurious variationbetween said control electrode and said principal electrode, saidrepeated variations and said spurious variations being impressedtogether when said repeated variations are present and said spuriousvariations being impressed alone when said repeated variations are notpresent, a time constant network having a time constant substantiallylonger than the longest of said periods of said repeated variation,means connected to said principal electrodes in current transmittingrelationship to said network, said connecting means including means forsuppressing any direct current component of the potental appearingacross said principal electrodes and also including means impeding theflow of current from said principal electrodes to said network so thatpotential of a substantial magnitude dependent on the amplitude of saidrepeated variations is built up on said network when said repeatedvariations are appreciably present and the potential on said networkremains low in the absence of said repeated variations, an output valvehaving a control electrode and principal electrodes, means connected tosaid principal electrodes of said output valve for impressing betweensaid principal electrodes a nonvarying energizing potential, and meansconnecting said network between said control electrode and one of saidprincipal electrodes of said output valve to impress the potential onsaid network in controlling relationship between said last-named controlelectrodes and principal electrodes to control the conduction of saidoutput valve.

3. An amplifier for use with apparatus which operates responsive torepeated potential variations having generally predetermined periods asdistinct from spurious potential variation, the operation of saidapparatus to be stopped on the disappearance of said repeated potentialvariations, the said amplifier comprising an input valve having acontrol electrode and principal electrodes, means connected to saidprincipal electrodes for impressing a substantially non-varyingenergizing potential between said principal electrodes, means connectedto said control electrode and one of said principal electrodes forimpressing both said repeated variations and said spurious variationbetween said control electrode and said principal electrode, saidrepeated variations and said spurious variations being impressedtogether when said repeated variations are present and said spuriousvariations being impressed alone when said repeated variations are notpresent, a time constant network having a time constant substantiallylonger than the longest of said periods of said repeated variation,means connected to said principal electrodes in current transmittingrelationship to said network, said connecting means including means forsuppressing any direct current component of the potential appearingacross said principal electrodes and also including means impeding theHow of current from said principal electrodes to said network so thatpotential of a substantial magnitude dependent on the amplitude of saidrepeated variations is built up of said network when said repeatedvariations are present and the potential in said network remains low inthe absence of said repeated variations, said connecting means alsoincluding voltage increasing means so that said magnitude issubstantially higher than said amplitude of said repeated variations, anoutput valve having a control electrode and principal electrodes, meansconnected to said principal electrodes of said output valve forimpressing between'said principal electrodes a non-varying energizingpotential, and means connecting said network between said controlelectrode and one of said principal electrodes of said output valve toimpress the potential on said network in controlling relationshipbetween said last-named control electrode and principal electrode tocontrol the conduction of said output valve.

References Cited by the Examiner UNITED STATES PATENTS 2,763,838 9/56McConnell 328149 2,833,938 5/58 Pinckaers 30788.5 2,992,340 7/61 Floyd328-446 JOHN W. HUCKERT, Primary Examiner.

ARTHUR GAUSS, Examiner.

2. AN AMPLIFIER FOR USE WITH APPARATUS WHICH OPERATES RESPONSIVE TOREPEATED POTENTIAL VARIATIONS HAVING GENERALLY PREDETERMINED PERIODS ASDISTINCT FROM SPURIOUS POTENTIAL VARIATION, THE OPERATION OF SAIDAPPARATUS TO BE STOPPED ON THE DISAPPEARANCE OF SAID REPEATED POTENTIALVARIATIONS, THE SAID AMPLIFIER COMPRISING AN INPUT VALVE HAVING ACONTROL ELECTRODE AND PRINCIPAL ELECTRODES, MEANS CONNECTED TO SAIDPRINCIPAL ELECTRODES FOR IMPRESSING A SUBSTANTIALLY NON-VARYINGENERGIZING POTENTIAL BETWEEN SAID PRINCIPAL ELECTRODES, MEANS CONNECTEDTO SAID CONTROL ELECTRODE AND ONE OF SAID PRINCIPAL ELECTRODES FORIMPRESSING BOTH SAID REPEATED VARIATIONS AND SAID SPURIOUS VARIATIONBETWEEN SAID CONTROL ELECTRODE AND SAID PRINCIPAL ELECTRODE, SAIDREPEATED VARIATIONS AND SAID SPURIOUS VARIATIONS BEING IMPRESSEDTOGETHER WHEN SAID REPEATED VARIATIONS ARE PRESENT AND SAID SPURIOUSVARIATIONS BEING IMPRESSED ALONE WHEN SAID REPEATED VARIATIONS ARE NOTPRESENT, A TIME CONSTANT NETWORK HAVING A TIME CONSTANT SUBSTANTIALLYLONGER THAN THE LONGEST OF SAID PERIODS OF SAID REPEATED VARIATION,MEANS CONNECTED TO SAID PRINCIPAL ELECTRODES IN CURRENT TRANSMITTINGRELATIONSHIP TO SAID NETWORK, SAID CONNECTING MEANS INCLUDING MEANS FORSUPPRESSING ANY DIRECT CURRENT COMPONENT OF THE POTENTAL APPEARINGACROSS SAID PRINCIPAL ELECTRODES AND ALSO INCLUDING MEANS IMPEDING THEFLOW OF CURRENT FROM SAID PRINCIPAL ELECTRODES TO SAID NETWORK SO THATPOTENTIAL OF A SUBSTANTIAL MAGNITUDE DEPENDENT ON THE AMPLITUDE OF SAIDREPEATED VARIATION IS BUILT UP ON SAID NETWORK WHEN SAID REPEATEDVARIATIONS ARE APPRECIABLY PRESENT AND THE POTENTIAL ON SAID NETWORKREMAINS LOW IN THE ABSENCE OF SAID REPEATED VARIATIONS, AN OUTPUT VALVEHAVING A CONTROL ELECTRODE AND PRINCIPAL ELECTRODES MEANS CONNECTED TOSAID PRINCIPAL ELECTRODES OF SAID OUTPUT VALVE FOR IMPRESSING BETWEENSAID PRINCIPAL ELECTRODES A NONVARYING ENERGIZING POTENTIAL, AND MEANSCONNECTING SAID NETWORK BETWEEN SAID CONTROL ELECTRODE AND ONE OF SAIDPRINCIPAL ELECTRODES OF SAID OUTPUT VALVE TO IMPRESS THE POTENTIAL ONSAID NETWORK IN CONTROLLING RELATIONSHIP BETWEEN SAID LAST-NAMED CONTROLELECTRODES AND PRINCIPAL ELECTRODES TO CONTROL THE CONDUCTION OF SAIDOUTPUT VALVE.